1. Field of the Invention
The present invention is related to a magnetic sensor comprising a SQUID made of superconducting thin film.
2. Description of Related Art
A SQUID is a fundamental device that uses technology for profit of superconductivity. SQUID can be used as a magnetic sensor having very high sensitivity.
Generally, a SQUID is produced by patterning a superconducting thin film. A pattern of superconducting thin film of the SQUID includes a washer pattern and a pair of terminal portions, and at least one weak link exists in the washer pattern. Superconducting current spreads in the washer pattern. Output voltage of the SQUID changes according to intensity of magnetic flux passing inside the washer pattern.
Actually, a shape of the washer pattern is generally quadrangle. There is a hole pattern in the center of the quadrangle pattern. The terminal portions grow from a pair side of the quadrangle pattern toward the outside. A pair of slit patterns exist on the other sides without the terminal portions. The weak links are located between the hole pattern and the slit patterns.
Various kinds of developments concerning the pattern of superconducting thin film of SQUID have been proposed.
There is a proposal in Jpn. J. Appl. Phys., Vol.32, p.662-664 (1993) concerning a SQUID that has a large outside diameter. On this SQUID, line width of the washer pattern is very large and focusing effect of magnetic flux occurs therein. As a result, density of magnetic flux passing inside of the SQUID becomes high, and sensitivity as a magnetic sensor becomes high.
By the way, an individual SQUID can be used as a magnetic sensor. However, actually, there are many cases that SQUID is used with a flux transformer. There is a proposal in Appl. Phys. Lett., Vol. 66, p.373-375 (1995). The flux transformer can be produced as a patterned superconducting thin film, too. Because of use of the flux transformer, sensitivity as magnetic sensor becomes high and physical layout of a magnetic sensor becomes easy. A detection object of a SQUID is magnetic flux passing inside of the SQUID. At the same time, said slit patterns to form weak links on the washer pattern opens toward outside of the SQUID at their ends. Therefore, some of magnetic flux that has been taken by the SQUID is leaked out through the slit pattern.
There is a proposal in Appl. Phys. Lett., Vol. 57, p.727-729 (1990) concerning this point. According to the document, one end of the hole pattern is enlarged and weak links are arranged outside of the washer pattern. When the flux transformer is combined with this SQUID, the weak links are located outside of the input coil. As a result, leak of magnetic flux is reduced.
There is other proposal in Appl. Phys. Lett., Vol. 63, p.2271-2273 (1993) concerning the same subject. According to the document, shape of the hole pattern becomes a rectangle and slit patterns are shortened. As a result, leak of magnetic flux is reduced. However, when the hole pattern becomes large, another problem occurs. When the area of the hole pattern increases, effective magnetic flux capture area (A.sub.eff) becomes large. At the same time, inductance of the SQUID becomes high. As a result, magnetic flux resolution of the SQUID falls down and total performance of the SQUID becomes low.
There is a proposal in Advances in Superconductivity VI Springer-Verlag, p.1119-1122 concerning this point. The hole pattern is formed as a thin and long rectangle. Degradation of magnetic flux resolution is supplemented.
There are other proposals in Jpn. J. Appl. Phys., Vol. 32, p. 662-664 (1993) and Appl. Phys. Lett., Vol. 63, p. 366-368 . According to the documents, slit patterns are covered by a superconducting thin film cover insulated from a SQUID and leak of magnetic flux decreases. Furthermore a part of the hole pattern is covered with the superconducting thin film cover and output voltage (V.sub.pp) rises.
There is another proposal in J. Appl. Phys., Vol. 73, No.11, p. 7929-7934 (1993). According to the document, a damping resistance is connected in parallel to the inductance of the SQUID and output voltage (V.sub.pp) of SQUID becomes high.
Furthermore, there is another subject concerning a magnetic sensor that it is very difficult to form a weak link by only patterning a superconducting thin film. An artificial grain boundary can be formed by way of deposition of an oxide superconducting thin film on a substrate having step. An artificial grain boundary can be made by even a method to use bi-crystal substrate or by bi-epitaxial growth. Weak link of artificial grain boundary is mentioned in Jpn. J. Appl. Phys., Vol. 32, p. 662-664 (1993). According to the document, an artificial grain boundary is formed in a straight line.